Large area deposition in high vacuum with high thickness uniformity

ABSTRACT

The invention relates to an effusing source for film deposition made of a reservoir comprising one hole characterized by the fact that the hole diameter is less than one order of magnitude than the mean free path of the molecules determined by the pressure and its thickness is at least one order of magnitude smaller than the diameter. Preferably the source has several holes.

FIELD OF THE INVENTION

The present invention relates to thermal or laser-assisted (electron orion beam-assisted) film deposition with chemical precursors in themolecular regime.

STATE OF THE ART

The business and technology of large area vacuum coatings have madesignificant progresses in the last two decades and are still proceedingwith great expectation for the future [1, 2]. In particular, we canmention typical applications as architectural and automotive glasses,solar cells, and micro-optoelectronics applications that require largerand larger substrates to reduce costs.

It is commonly believed that the alternative approach of vacuumdeposition is difficult to scale up to industrial applications.Furthermore, vacuum deposition is also known to be inherently moreexpensive and difficult with these two drawbacks increasing as lowerpressures are aimed at. However the use of vacuum deposition has someadvantages for industrial applications. Indeed, high-throughputtechniques, such as MOCVD, can produce large quantities of materials,while the precise in-situ control of thickness, purity and compositionthat can be achieved through MBE has been invaluable in developing thetechnology. A hybrid technique (called CBE, MOMBE, or GSMBE), consistingin molecular beams of CVD precursors, could provide a good solution:high control, possibility to grow good quality devices and higher growthrates than those typically obtained in conventional MBE. For thesereasons, by 1990 most of the major electronics and telecommunicationcompanies had a research effort in Chemical Beam Epitaxy [3].

Several patent documents discuss the increase of deposit uniformity bothin thickness and in chemical composition. For vacuum deposition withchemical precursors, two precursor transport regimes have to bedistinguished: Chemical Vapour Deposition (CVD) in the viscous or thetransition regime and Chemical Beam Epitaxy (CBE) in molecular regime(also called Metal-Organic Molecular Beam Epitaxy (MO-MBE) or Gas SourceMBE (GSMBE)). As a general rule, most of the patents are for CVD systemsto solve the classical problems reported in literature. In the molecularregime very few patents have been deposited.

As a summary, we can report the following points:

-   -   CVD processes have a much higher production capacity, but suffer        from poor utilization efficiency of the expensive and toxic        reactants gases (L. M. Fraas, in U.S. Pat. No. 4,828,021). In        opposition, MBE devices are not easily scalable to meet the        needs of the growing market.    -   It should be desirable to have an apparatus and a process that        could be extended to the general use of any metal organic gas in        a vacuum environment to optimise the growth of uniform        semiconductor layers on large area substrates while minimizing        the disposal problems of the toxic reactant gases and the        reaction by-products.    -   A greater degree of control over uniformity of coating and/or        etch rate, with an ability to alter uniformity profiles in a        selective and controlled manner, is required in deposition        techniques (J. J. Schmitz reports U.S. Pat. No. 5,387,289).    -   Different gases in the source or in reaction chamber are liable        to react very rapidly with each other (i.e. arsine and        triethyl-indium). They cannot be mixed in advance and should be        introduced simultaneously, but separately. Concentric annular        arrays of sources have been proposed to provide different gases        (Frijlink U.S. Pat. No. 4,961,399), but several problems are        still open.    -   Rotation, or planetary motion, was also introduced to provide        uniform thickness (Frijlink U.S. Pat. No. 4,961,399), but light        processes are incompatible with such a motion.    -   Plurality of vapour streams to generate partially overlapping        molecular angular distributions has been proposed (Chuong Van        Tran U.S. Pat. No. 6,082,296). Zinck reports a source with an        array of micro-channels to use as a gas source for MBE (U.S.        Pat. No. 5,188,671). With this over-cosine collimated source,        the growth rate can be increased as the local pressure at the        surface of the wafer can be over 100 times higher than the        residual pressure.    -   A state of the art is given by Tanabe for vacuum coatings of        large area substrates and high precursor efficiency use (patent        EP 0 982 411 A2). This is the closest result found for patent to        our proposed invention, but uniformity is only about 5-10% and        is hence not suitable to many applications.

What can be said is that no global solution has been found up to now andthe above cited patent document have only solved some of the specificmentioned problems.

The following points and basic understanding are reported by non-patentliterature. In particular, viscous and transition regimes are poorly, ifnot at all, compatible with light (electron ion)-assisted processes.Furthermore, flexibility is poor to achieve good thickness uniformity(inter-correlation of deposition parameters), while up-scaling to largersubstrates or upgrading to new materials is not straightforward and canbe very difficult. We will hence focus on the molecular beam regime.

In the molecular regime, gas-phase collisions are assumed negligiblecompared to molecules to walls collisions. Equivalently, we can say thatthe mean free path of molecules is large compared to the chambergeometry. This line of sight travelling of molecules leads to exactmodelling of their distribution on whatever surface is used as adeposition area and is independent of parameters such as substratetemperature or precursor molecular species. Due to the closed relationwith light propagation, several authors have compared particle effusionwith light in a diffusive medium [6, 7]. In molecular beam conditions,we can suppose to be in conditions that are similar to a verytransparent medium for light. In particular, the gas inlet system,consisting in one or more effusive sources with given molecular angulardistribution and relative position compared to the deposition area, canbe assumed theoretically as the unique parameter influencing molecularimpinging rate distribution on the deposition area [8, 9].

Experimentally however, several effects can modify this view [10]. Amongthese effects, we can list vapour pressure uncertainties, the cosineemission law from walls when the walls are not totally inert against thevapours (surface diffusion [11] and specular reflexions of molecules onsurfaces [12]). These effects are however very limited to the workingtemperatures below 100-200° C. required avoiding chemical precursoradsorption and sufficient vapour pressure. Another effect that can makefail attempts to produce uniform impinging rate on the deposition areais that molecules can bounce on walls inside the deposition chamber.These scattered molecules would hence lead to uncontrolled deposition.This drawback can be reduced with liquid nitrogen cryopanels that willcondense both unused precursor and by-products. Analytical modelling[13, 14], Monte Carlo simulations [15-17], and experimental work [9, 18,19] have been used extensively to achieve parameters optimisation. Verygood agreement is usually achieved between experiments, simulations andanalytical modelling if the previously described precautions are taken.

Sources flow angular distribution and their relative position to thedeposition area can hence be assumed, practically and not onlytheoretically, as the key parameter to achieve high uniform thickness.Effusive sources are defined as apertures between the main chamber, inwhich precursor molecules in the form of molecular beams will impinge onthe deposition area, and a reservoir (in our case a single pre-chambercommon to all the sources for a given precursor) in whichhigher-pressure conditions occur ranging from the viscous, through thetransition, to the molecular regime. Evaporation sources are used inphysical vapour deposition processes and generally require very hightemperatures necessary to reach sufficient vapour pressure, while gassources use volatile chemical precursors resulting in high enoughpressures even at temperatures as low as 100° C.

The difficulty in the molecular flow regime is to achieve simultaneouslyboth high thickness uniformity and high growth rates [4], which areusually related to high efficiency use of precursor [4, 5], and highinitial investment costs [1] as reducing the size of the reactor usuallyallows reduced equipment costs, but also leads to poor thicknessuniformity.

An important aspect to be considered in the deposition process usingeffusive sources is the growth rate, which is tightly related to thesource flow throughput. This flow can be assumed proportional to theaperture area of the source and to the vapour pressure of the gas at agiven temperature even if some exceptions exist to this rule (forexample collisions inside small capillaries reducing effusion [20]).

Large aperture sources are usually required both in physical evaporationand gas sources configuration. The effusing area can however be either asingle large area source or distributed among several smaller sources.Small apertures have not been used very often in physical vapourdeposition sources because difficulties appear due to regulation of eachsource. These sources are furthermore expensive and unstable as thevapour condensation and evaporation can modify the effusion. Gas phasesources are user-friendlier and allow the use of smaller sources withoutthe described drawbacks, but have only been investigated recently.

The advantage of small sources lays in the more precise control of flowthroughput as discussed by Vassent [10]. The limit is given for idealsources (very small effusion orifices compared to molecular mean freepath related to pressure (about 0.5 mm at 10⁻²-10⁻³ mbar)) by thepressure (temperature) control of the cell. In large aperture cellsthermal equilibrium is not warranted, while small apertures, like inKnudsen effusion, work as thermal blackbody.

Another reason that makes small gas sources more attractive is thatsources are generally tilted to the normal of the deposition area toincrease uniformity [21]. Physical reasons may limit this tilting anglethat is not always optimal for evaporation sources due to liquidgeometry level considerations [5]. Finally, smaller sources compared toa single large source are interesting as light induced or light enhanceddeposition is incompatible with substrate motion. For sources that arestable over the time interval required for a few revolutions of thesubstrate, substrate rotation is equivalent to a continuous ring ofidentical sources [18].

In addition to the growth rate and geometrical distribution of sourcesas discussed here above, another important parameter has to bediscussed: the angular distribution of molecules effusing from thesource. Literature on the subject is extensive from first works ofKnudsen at the beginning of the XX^(th) century and will not be reportedhere. A complete review is given by Stekelmacher to understand effusionmechanism and the different effusion sources in different regimes [22].

The first quality of a source with respect to angular distribution isreproducibility and stability. One of the problems in MBE sources isthat the source angular distribution may vary to a large extent as afunction of the filling level. This leads to poorly controllable angulardistributions of effusing molecules with time [23]. Gas sources aregenerally more reliable, but care must be taken to control molecularangular distribution that can depend on pressure. Another error that canaccount for different angular distributions is that the source dimensionaperture can have a deep impact on the molecular distribution. Inopposition to the previously discussed case where the orifice aperturewas compared to the mean free path, the source aperture geometry mustnow be compared to the distance between the source and the depositionarea. Leiby has studied this for molecules effusing from circular andrectangular apertures [14]. He demonstrated that for large distances ofthe source from the deposition area compared to the source diameter,every source can be considered as a point source. Point sources are moreeasily dealt with as they can be assumed as ideal sources (the angulardistribution is constant as a function of the distance to the depositionarea). Winkler [24], for example, reports a formalism to consider nonpoint-like circular sources that follows a procedure previouslydescribed by Olander [25].

Point sources can arbitrarily be subdivided into three classes asreported by Cale [8]: cosine sources (Knudsen sources) over-cosine andunder-cosine sources depending on their degree of collimation. The mostinvestigated sources are the over-cosine sources [22] [26] mainlybecause they can reach higher growth rates [4] and optimise precursoruse. However, such collimated sources lead to several problems. Amongthese we can list a general increase in source-deposition area distanceto keep uniform impinging rates and higher sensitivity to misalignments[10]. Under-cosine sources in opposition to more focused sources havebeen poorly discussed in literature.

The obtained results reported by the scientific community with regard tothickness uniformity deposition are in general difficult to evaluate ifthe parameters of reactor size versus substrate size are introduced.Uniformity of a few percent can be achieved with a single source and arotation of the substrate. An order of magnitude improvement in the filmthickness uniformity can be obtained with planetary motions of thesubstrate or by introducing suitable masks [27] or both. However,thickness uniformity is closely related to the deposition area diameter,the precursor efficiency use, and growth rates [4]. As a general rule,thickness uniformity decreases rapidly with deposition increasing areaand with reduced distance of the sources to the deposition area. Jahanet al. report a thickness uniformity of about 3% on a 2-inch diametersubstrate [28]. Aers reports a uniformity of 3-4% over a 3-inchsubstrate [29]. Spring Thorpe achieved a thickness uniformity of 3% on a76 mm diameter substrate, but uniformity quality degrades rapidly to 20%as the crucible empties [5]. All these references however, do not reportany source-deposition area distance neither an evaluation of the ratiobetween the molecules impinging on the deposition area compared to thetotal effused molecules, which are important parameters for an optimizedcost effective deposition system. A systematic approach and a goodunderstanding of these parameters are still lacking.

SUMMARY OF THE INVENTION

The invention relates to a novel small point gas source (using chemicalprecursors) for vacuum deposition in the molecular regime able to leadto highly uniform thickness on large areas with small reactor size andhigh precursor efficiency use. The source design is also compatible withlight-assisted (electron or ion-assisted) deposition. As alternative todeposition, homogeneous etching can be achieved if it relies onimpinging rate of etching chemical on a given area.

The reactor design is such that the source relative position and itsangular distribution of effusing molecules are the only parametersaccounting for their distribution on the substrate. Because of the lineof sight propagation of the molecules in the molecular regime, thedistribution of impinging molecules on the substrate can be calculatedmathematically.

In particular, the reactor is composed of a precursor reservoir heated(for example by a thermo-regulated oil-bath up to temperatures of 200°C.) connected to a pre-chamber with a ring shape allowing irradiation ofthe substrate through its centre. Four or more sources, consisting inholes in a vacuum tight sheet separating the pre-chamber from thedeposition chamber, where the substrate is positioned, are responsibleof the molecular impinging distribution on the substrate. The effusingrate is controlled by regulating the pressure and the temperature of thepre-chamber resulting in only one single cost efficient system controlfor all the sources. All the system is baked at a temperature at least20° C. higher than the temperature of the precursor reservoir to avoidprecursor condensation. A cryo-panel is used to condense all themolecules effusing from the sources that do not collide directly on thesubstrate and of the by-products resulting from the chemicaldecomposition of the precursor. A pumping system is used to achieve avacuum between 10⁻¹² and 10⁻³ mbar in the deposition chamber and avacuum between 10⁻⁶ and 10 mbar in the pre-chamber.

Furthermore, the characteristics of the sources have been chosen for thefollowing reasons:

-   -   1. Gas sources:        -   Low temperatures in the effusing cell.        -   Multiple sources are easily achievable (cost effective            solution).        -   Small sources do not get clogged.        -   Stability in time.        -   All orientations are possible independently on the filling            level of the source.        -   Generally higher throughput.        -   Higher reproducibility.        -   Ideal conditions to model mathematically.    -   2. Small sources (small diameter compared to mean free path of        molecules in the pre-chamber):        -   Thermodynamic equilibrium is guaranteed.        -   Ideal to model mathematically.    -   3. Point sources (small diameter compared to distance of the        source to the deposition area):        -   No variation of angular distribution as a function of            distance to the deposition area.        -   Ideal to model mathematically.    -   4. Large aspect ratio between hole thickness and hole diameter:        -   No opaque regime that reduces effusion.        -   Effusing flow oriented normal to the surface. No            complications due to poor controlled holes orientation.    -   5. New ad hoc molecular angular distributions:        -   Non-focused sources to minimize misalignments effects on            thickness uniformity for deposition area.        -   Non-focused sources to minimize reactor size: a larger            acceptance angle is possible compared to more focused            distributions.        -   Reduced precursor flow outside the deposition area (i.e. for            angles lower and/or greater than critical angles (asymmetric            sources)).    -   6. Single pre-chamber connected to all the effusing sources        -   Simple and reproducible        -   Cost effective solution        -   Graded composition possible by precursors inter-diffusion    -   7. Disposition of the sources on one or more rings with possibly        a tilt between the rings        -   Avoid substrate rotation and/or planetary motion        -   Different materials co-deposition with controlled            composition and thickness uniformity

DETAILED DESCRIPTION OF THE INVENTION

A. The Effusion Source

A typical effusion source is a hole of 0.5 mm drilled in a foil of 0.05mm of thickness between a reservoir (pre-chamber) with a pressure of10⁻³-10⁻² mbar and the deposition chamber with a pressure below10⁻³mbar. The hole dimensions however, depend on the pressure in thepre-chamber and on the substrate size, and could vary from 0.001 and 50mm. Furthermore, the thickness of the hole is about one order ofmagnitude (or more) smaller than the diameter, while the distance of thesource to the deposition area is one order of magnitude (or more) largerthan the diameter of the hole.

B. Disposition and General Description of the Sources

The combination of several sources may allow substrate rotationavoidance. Several holes are uniformly distributed on an annulargeometry (see FIG. 1). The formula that describes the distribution ofimpinging molecules on a planar surface for several cos^(n)distributions of the effusion sources distributed on a ring is thefollowing:${I(\vartheta)} = {\left. {I_{0}{\cos^{n}(\vartheta)}}\Rightarrow I_{tot} \right. = {I_{0}{\sum\limits_{i = 1}^{p}\left\lbrack \frac{h}{\sqrt{h^{2} + R^{2} + r^{2} - {2\quad{Rr}\quad{\cos\left( {\beta - {i\frac{2\pi}{p}}} \right)}}}} \right\rbrack^{n}}}}$

For Knudsen effusion sources n=4 (cosine sources).

To illustrate the concept of reactor size reduction with opportunethickness uniformity, several examples of angular distribution shaping(as reported in the summary of the invention point 5) are provided inTable-1. Analytical modelling of precursor transport is applied to findthe adequate parameters providing molecular impinging uniformity betterthan 1% on a 150 mm substrate for various arbitrary cos^(n)distributions, distance h of the sources to the substrate, and radius Rof the ring on which are distributed the sources (see Table-1). With theparameters reported in Table-1, the distributions are identical with anerror less than 0.1%. These cost sources (with n<4 under-cosine sources)are not existing sources nor are they to be considered optimal sources,but are used only to show that the trend of the reactor size is reducedwith decreasing the focusing of the source. TABLE 1 Geometricaldisposition of the holes (both R, h varied) as a function of theeffusing beam angular distributions (cos^(n)) to achieve uniformitybetter than 1%. Distribution Cos¹ Cos² Cos³ Cos⁴ Cos⁵ Cos⁶ Cos⁸ R value(mm) 99.2 113.2 119.57 122.4 124 127 129 h value (mm) 57 100 133 159181.5 205 242

Introducing a tilting angle (see FIG. 2), the dimensions can be furtherdecreased. The more general equation is:$I_{tot} = {I_{0}{\sum\limits_{i = 1}^{p}\left\lbrack \frac{{- {x_{s}\left( {x - x_{s}} \right)}} - {y_{s}\left( {y - y_{s}} \right)} + {h\sqrt{x_{s}^{2} + y_{s}^{2}}\tan\quad\Phi}}{\sqrt{\left( {\left( {x - x_{s}} \right)^{2} + \left( {y - y_{s}} \right)^{2} + h^{2}} \right)\left( {x_{s}^{2} + y_{s}^{2}} \right)\left( {1 + {\tan^{2}\Phi}} \right)}} \right\rbrack}}$

Similar distributions (within still an error of less than 0.1%) areachieved with parameters reported in Table-1 and Table-2. We can seethat the dimensions can be further reduced with a small tilting angle ofthe sources. Furthermore, several tilted concentric rings could beconsidered equivalent to planetary motion.

It can be shown that this angle is smaller and less critical onuniformity distribution for under-cosine sources. TABLE 2 Geometricaldisposition of the holes (R, h and Φ) for Cos¹ distribution to achievedistribution uniformity better of 1% on a 150 mm wafer. Tilt angle (π/2− Φ) 0° 3° 6° 9° R value (mm) 122.4 98 96 80.8 h value (mm) 57 48 3817.2

C. Molecular Beam Shaping

The second point discussed is how to achieve the desired distributionshaping of the sources required for the already discussed reasons inpoint 5 of the summary of the invention. In particular, under-cosinedistributions for small angles and over-cosine distributions for greaterangles corresponding to regions outside the substrate (see FIG. 3) areaimed at. A tilt angle could be considered the first step in molecularbeam modification (relative to the substrate), but this method is verylimited. Two other solutions are proposed, as examples, but should notbe considered exhaustive.

First Source Design

The first design is based on selecting and promoting molecules escapingthe source with a given angle. Two different types of molecules willescape the source: the molecules that had the last collision inside thepre-chamber with another molecule and those that had the last collisionon a surface inside the source.

On one hand, a volume below the effusing aperture is a forbidden regionfor gas phase collision. On the other hand, a pumping aperture will actas a trap for surface scattered molecules. Counterbalancing both effectscould lead to shaped distributions. Furthermore, variable pressureconfiguration could lead to variable angular distributions without anymoving part or modifications of the set-up.

An example to reduce molecules effusing at small angles is reported inFIG. 4 with a cone-like shaped forbidden volume. Any kind of structurecould serve to reach this purpose. In the cone-case, the apex may be cutand a hole provides a pumping aperture. If the cone is positioned underthe hole at a distance b≠0, we will have a progressive increase of thevolume, as angle will increase that will depend on the ratio between thedistance b and the mean free path λ. In particular, variable pressureconfiguration will lead to variable effect of the cone as the mean freepath is changed. A particular case of this configuration could be anegative parameter b; i.e. the cone exits the pre-chamber through thehole. As multiple sources are to be used, mechanical complexity ofmultiple cones can be avoided by producing a continuous structure.

Second Source Design

As a general rule, any kind of molecular angular distribution can beachieved by opportune disposition of several holes a 3-D surface.However, only in the case in which these sources are dispatched closetogether compared to their distance to the deposition area we canconsider them as a single point source with the advantage of easymathematical modelling. Furthermore, the total area of the holes shouldbe small compared to the area separating the 3D surface from thepre-chamber to avoid gas depletion and pressure gradients. Furthermore,each single hole must satisfy the rules introduced previously in thesummary of the invention.

A particular shape of interest is a hemisphere (see FIG. 5). Inparticular asymmetric sources are easily produced leading to gas wastereduction.

FIGURES SUMMARY

FIG. 1: Disposition of effusion holes with annular geometry (1). R isthe radius of the ring on which are distributed the holes, r is thedistance from the centre on the substrate (2), and h is the distance ofthe substrate from the holes containing plane (3).

FIG. 2: Φ is the tilt angle of the surface on which is the source S andis oriented towards the z-axis.

FIG. 3: Shape of the ideal angular distribution (1), to achieve highimpinging rate uniformity and small reactor size, compared to Knudseneffusion (2). Both curves are normalised to achieve same number ofeffusing molecules. In the ideal curve (1), 60° is assumed to be thecut-off plane that discriminates between deposition on and outside thedeposition area. A rapid decrease of the molecules occurs after thiscritical angle and the under-cosine distribution (4) is modifiedresulting in an over-cosine distribution (3). Asymmetric sources couldalso prove useful.

FIG. 4: The source is composed of a hole (1), a volume (2) that reducethe region where gas phase collisions are allowed, and a pumpingaperture (3) that reduce the surface from which scattered molecules canexit the source. Gas phase collisions are restricted to larger angles inFIG. 4 a. Surface scattered molecules are reduced for small angles FIG.4 b. When the parameter b is not null, a variation in pressure induces avariation of the mean free path λ. A different contribution of the coneto angular distribution is hence achievable as a function of pressure inFIG. 4 c. The structure can also exit the source as reported in FIG. 4d.

FIG. 5 a: Fractal source composed of a distribution of effusing holes ona hemispherical surface.

FIG. 5 b: Asymmetric fractal source with preferential orientation of themolecular beam.

BIBLIOGRAPHY

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1-12. (canceled)
 13. An effusing source for film deposition comprising asingle hole or a combination of holes having a hole diameter less thanone order of magnitude smaller than the mean free path of the moleculesand having a thickness at least one order of magnitude smaller than thediameter, wherein the combination of holes are distributed on a ring oron several rings with a tilt angle, and wherein the effusing sourcegeometry is modified to control and shape molecular beams impingingdistribution on a substrate under vacuum.
 14. An effusing sourceaccording to claim 13 designed to impinge effusing molecules on adeposition area situated at a distance of at least one order ofmagnitude larger than the hole diameter.
 15. An effusing sourceaccording to claim 13 designed in such a way that the ratio “number ofmolecules impinging on a deposition area” versus “total number ofeffusing molecules” is more than 20%.
 16. An effusing source accordingto claim 13 wherein the source is designed in such a way that thedistribution of impinging molecules on a deposition area in themolecular regime only depends on the source relative position and on itsmolecular angular distribution.
 17. An effusing source according toclaim 13 wherein it includes a gas phase collisions forbidden volume andpumping apertures in the surface of said volume to reduce thecontribution of walls surface scattered molecules for certain angles.